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Old 3rd December 2005, 02:59 AM   #11
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Putting the regulator in front of the control transformer has two problems. The regulator must work on AC, this is not common although it could be done. And this will do nothing to help the loss of regulation caused by the transformer itself.

I am currently using 5 power supplies on my bench. The Knight shown in the picture. This is a tube regulated 0 to 400 volt 200mA power supply with 2 sets of 6.3 volts and bias. a PACO which is a slightly lower powered Knight. Two low voltage medium current supplies for filaments, and solid state stuff.

I also have the DIY version which is a quick and dirty variac, transformer, diode bridge, capacitor power supply. Since that schematic was drawn, I added a choke and a second cap. Now each channel has CLC filtering. Each channel can be used seperately (for circlotrons) or wired in series for a 0 to 600 volt supply. The additional filtering has cleaned up this supply so that I can use it on SE amps.
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Old 3rd December 2005, 04:01 PM   #12
anatech is offline anatech  Canada
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Hi tubelab,
If you rigged up an optocoupler and stuck the pass elements in a bridge rectifier, you would have linear regulation for the control transformer. Importantly the transformer gets a sine with small crossover distortion. Compensation for transformer regulation is then achieved. Look at a Revox capstan motor for the basic principle.

Otherwise, even your variac to transformer and rectifier is sometimes all you really need.

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Old 3rd December 2005, 08:08 PM   #13
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OK -- here's the supply I put into a Heath IP-17 --
Click the image to open in full size.
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Old 5th December 2005, 02:31 AM   #14
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Interesting... could you also please post the schematic? I'm interested and I'm sure that others are as well.

Thanks!

Mark
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Old 5th December 2005, 12:09 PM   #15
ilimzn is offline ilimzn  Croatia
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Default Re: DIY Bench PS

Quote:
Originally posted by Valverine
1. Two HT channels, up to 600V, variable. One for driver stage - 100 ma is plenty; the second for output stage, higher current.
Well, I'm kind of in the same place you are regarding a benchtop PSU

If you ever use tetrodes / pentodes, a regulated screen grid supply is a good idea, but you can take that from the main reguated output.

Quote:
2. A bias supply, up to 120V (this is a usual variac range)
Fortunately, this is a low current output...

[/quote]
3. Two heater supplies, up to 12.6V, variable, few good amps rating.
[quote]

This is where you might be able to use a recycled switchmode power supply, assuming it is a so-called flyback type. These can be relatively easily modified for different output voltages, and are capable of constant power operation (maximum current is lower at higher output voltages). A current limiter would be a good idea, so you can also use series connection heaters, in constant current mode. Heater supplies also need to be floating.

Quote:
1. It looks like the limiting factor in that circuit is the regulator IC; if I substitute it for one used in jackinnj (e.g., IRFPG40) what is the right way to re-calculare the other component values?
IRFPG40 is a MOSFET, not an IC. In any case, yes, the series pass element is the limiting factor. Fortunately, there are a number of 100W 800V-1kV MOSFETs around. A good place to look for them is in old CRT monitors
There is also the problem of heat generated at lower output voltages. For isnatnce, if you wanted to test 6DJ8s,m you wuld need a 100V supply, which means your pass element would have to drop >500V at rated current. This can come out as a whole lot of Watts! For many reasons, I would never use a bipolar transistor as a pass element in such an application. There are also other measures that you can take. In general, you need lower currents at higher voltages. A design I am working on at t he moment uses a relay to either full-wave rectify the output of a 220V-220V isolation transformer, or work as a voltage doubler, depending on what output voltage is chosen. So, up to some 250V I can get up to 500mA out of the power supply, over 250V and up to 500V this drops to 250mA (which should be lerfectly fine for experimenting). The advantage is that the pass element never sees more than about 350V across it, thereby reducing the amount of generated heat.
I intend to use two 100W 2SK(something) MOSFETs reclaimed from a switchmode power supply, in parallel (on a fan cooled heatsink ).

Quote:
2. Is it a possibility to insert the regulating circuit between the variac and the control transformer (meaning the variac + transformer arrangement as at tubelab)? It looks that this way the regulator itself doesn't need to be that high voltage (120V max). Is there an additional benefit that the inductance of the control transformer will act as a smooting choke?
This is possible but not at all easy. The effective inductance of the transformer is actually negligible for this application.

Quote:
3. Is there a need in a smoothing choke or will regulator itself be a sufficient ripple-killer in case it goes after both transformers?
If you need a choke, you did not make a very good regulator

Further tips: it helps greatly if you have a separate winding on your HV transformer, to power the regulator circuitry. It needs to be well insulated - it has to 'float' and would be tied to the output of the regulator. This simplifies regulator circuitry CONSIDERABLY. Although a Maida regulator type could be used for this application, the required current through the voltage setting pots can be a big problem. With this sort of arrangement, a very small current can be used, making it easy to find a good pot.
I am fortunate to have several multiwinding transformers at my disposal, amongst them I have an extra 20V winding for exactly this purpose - though, of course, you could use a small extra transformer too.
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Old 6th December 2005, 02:33 AM   #16
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Default Re: DIY Benchtop PS

Hi ilimzn,

Thanks a bunch for your reply it is encouraging to know that somebody out there is "kind of in the same place"

Quote:
There is also the problem of heat generated at lower output voltages. For isnatnce, if you wanted to test 6DJ8s,m you wuld need a 100V supply, which means your pass element would have to drop >500V at rated current.
The idea is to drop the voltage on the variac first, then to regulate. Does it make sense?


Quote:
Further tips: it helps greatly if you have a separate winding on your HV transformer, to power the regulator circuitry.
This is a bit over my head why do I need an extra power source for this circuitry? I don't see any in the circuit that I linked above.

Anyway, will go to look up what "Maida regulator" is ...

Thanks!
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Old 6th December 2005, 12:00 PM   #17
ilimzn is offline ilimzn  Croatia
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Default Re: Re: DIY Benchtop PS

Quote:
Originally posted by Valverine
The idea is to drop the voltage on the variac first, then to regulate. Does it make sense?
Yes, but it requires a double control. Still, it should not be a big problem and certainly saves on heat. In my case it's a bit impractical since I intend on having a dual separately regulated output PS, I would neet two variacs, which is a bit of overkill

Quote:
This is a bit over my head why do I need an extra power source for this circuitry? I don't see any in the circuit that I linked above.
The circuit you linked does have adjustable output but it is not regulated, and it does not filter hum any better than a simple retifier plus cap, i.e. has bad load and line rejection. Output impedance, about which the author makes a big fuss, has little to do with the MOSFET and lots to do with the impedance of the transformer winding - here we have to consider LF (close to DC) impedance as this translates directly to DC load regulation.

Once you start introducing real regulation into the schematic, you have a problem if your regulator circuits are ground referenced - everything in them will operate at maybe a dozen V, but the output has to drive a gate of a MOSFET, several hundred V above ground, and the input has to sense the PS output voltage, again, several hundred volts above ground. This former can be a problem, there needs to be some sort of active device (BJT, MOSFET, even tube) that spans this several hundred V gap.

Instead, you could reference the regulation circuitry to the output line, and it's power supply is tied to the output voltage, tracking it. Instead of having an output that has to span several hundred V, you can drive the MOSFET directly from the regulator circuitry (even directly from an OP-amp chip output!), and you only have one input which senses how far 'down' ground is, in volts, and regulates the output accordingly. Because the sense input is tied through a resistor that drops most of the output voltage (usually output minus reference, the former being a few V), in both topologies, this is not as critical.

The exception to this may be the Maida regulators that use a two-tier pass element: a MOSFET and a LM317 regulator IC. The LM317 requires a minimum current to work properly, which can be on the order of 10mA. Because the sense resistor divider works in such a way with this topology, that the current through it is constant, it is ofetn used as a 'default load' as well. This current flows through the adjustment pot and the pot has essentially the full output voltage across it - figure 500V maximum, at 10mA of current, and you get 5W of heat - this is quite a problem for most potentiometers. The Maida regulator also has another problem - adjustable current limit is not easy to implement - and in fact, to implement it properly, you need a separate floating power supply for something like an OP-amp chip.
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Old 6th December 2005, 12:53 PM   #18
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fwiw -- this is the circuit I use (almost) -- the actual one pictured several posts ago is a bit of a Rube Goldberg (1) as I worked with a dual DAC for the B+ and C- and with an assortment of opamps got it to track pretty well. Since voltage and current were measured instantaneously I didn't regulate that supply. This is the basic design from H&H -- without the current limiter and with a DAC instead of a fixed reference. (Note -- you can see the fan in this supply -- I used a nibbling tool to cut out a hole for a small computer fan.)

(1) a "Rube Goldberg" refers to an engineering creation which is overly complex -- from the cartoon character in the 1930's ! I think this term may be out of circulation in the lingua franca of our younger readres.
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Old 6th December 2005, 01:32 PM   #19
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Default Re: Re: Re: DIY Benchtop PS

Quote:
Originally posted by ilimzn
you need a separate floating power supply for something like an OP-amp chip.
you can use an LM10 -- not a particularly fast chip but it can be used where a floating reference is required. You can always use zener's to float the opamp -- just make sure that there is a diode to protect it from an overvoltage situation (i.e. you have to clamp the inputs).
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Old 6th December 2005, 01:36 PM   #20
Sherman is offline Sherman  United States
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Quote:
Originally posted by jackinnj
...
(1) a "Rube Goldberg" refers to an engineering creation which is overly complex -- from the cartoon character in the 1930's ! I think this term may be out of circulation in the lingua franca of our younger readres.

My 18 year old son came home the other day and asked if I had ever heard of "Rude Goldberg"! I told him I had heard of "Rube " Goldberg and asked what the context was. Apparently in his physics class they have to build a machine to do a simple task but the machine has to go through at least six steps to accomplish what could be done in one and his teacher told them it was a Rube Goldberg type of thing.

So at least a few of the younger ones are being introduced to the idea of a Rube Goldberg contraption!
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